Bogota - US President Donald Trump said Wednesday that he formally recognises Juan Guaido, the head of Venezuela's opposition-dominated National Assembly, as the country's interim president.

Guaido declared himself Venezuela's interim president during anti-government rallies in Caracas.

"Today, on January 23, I swear to formally take over the competencies of the National Executive as president of Venezuela to achieve the end of the usurpation," Guaido told thousands of supporters in the capital.

The crowd sang the national anthem and broke out in applause. Protest rallies estimated to include hundreds of thousands of people demonstrated against the country, calling on President Nicolas Maduro to resign.

Making his own announcement shortly thereafter, Trump called on other Western governments to recognise Guaido's announcement as he vowed to keep up pressure on Caracas.

"Today, I am officially recognising the President of the Venezuelan National Assembly, Juan Guaido, as the Interim President of Venezuela," Trump said in a statement, referring to the legislative body as the "only legitimate branch of government."